It is well known that low-back pain is one of the most frequently occurring and expensive disabling ailments, especially for patients in the 30-60 year age bracket. Although low-back pain syndrome is a very common occurrence, its diagnosis to this day is very difficult.
The vertebral column (spine) is a biomechanical structure composed primarily of ligaments, muscles, vertebrae and intervertebral discs. The biomechanical functions of the spine include i) support of the body (trunk and appendages), which involves the transfer of the weight and the bending movements of the head, trunk and arms to the pelvis and legs, ii) complex physiologic motion between these body parts, and iii) protection of the spinal cord and the nerve roots.
The major regions of the spine are the cervical, thoracic, lumbar and sacral. The vertebrae increase in size and mass from the cervical to the lumbar regions. The increase in size of the vertebrae is directly related to an increased capacity for supporting larger loads. The lumbar region is therefore the major load bearer of the spine. However, this increase in load bearing capacity is paralleled by a decrease in flexibility. Because the lumbar region bears heavier loads than other regions of the spine, the lumbar trunk (low back structure) is more susceptible to strain and hence low-back pain.
The spine is comprised of a series of joints known as motion segment units (MSU). The motion segment unit is the smallest component of the spine that exhibits kinematic behavior characteristic of the entire spine. The motion segment unit is capable of flexion, extension, lateral bending and translation. The components of each motion segment unit include two adjacent vertebrae and their apophyseal joints (facet capsules), the intervertebral disc and the connecting ligamentous tissue. Each component of the MSU contributes to the mechanical stability of the joint.
The facet capsules, for example, are bony elements which help support compressive loads (approximately 20%) and resist torsional motion. The intervertebral disc, composed of the nucleus pulposus (a gel-like hydrodynamic ball bearing located at the center of the disc) and annulus fibrosus (concentric collagen fibers oriented at 30 degrees to one another and encapsulating the nucleus pulposus) gives the spinal column resilience and the ability to withstand compression, rotation and bending strains. Forces directed axially are absorbed by the nucleus pulposus and transmitted radially allowing for them to be absorbed by the fibers of the annulus fibrosus. In addition to the support provided by the facet capsules and the intervertebral disc, the ligaments, uniaxial elements which support loads in tension, are crucial in assuring the structural integrity of the spine. Each of the seven ligaments contained in a lumbar spine motion segment unit (intertransverse ligament, posterior longitudinal ligament, anterior longitudinal ligament, ligamentum flavum, capsular ligament, interspinous ligament, and supraspinous ligament) aids in assuring spinal stability by limiting excessive range of motion and absorbing energy that is applied as a result of trauma.
Many causes of low-back pain are attributed to the instability of the motion segment unit. According to A. A. White et al., "Spinal Stability: Evaluation and Treatment", American Academy of Orthopaedic Surgeons Instructional Course Lectures, Vol. 30, pp. 457-484, C. V. Mosbly, 1981, segmental instability is defined as "the loss of ability of the spine under physiologic loads to maintain relationships between vertebrae in such a way that there is neither damage nor subsequent irritation to the spinal cord or nerve roots, and, in addition there is no development of incapacitating deformity or pain due to structural changes". In other words, instability is an abnormal response to applied loads characterized by motion in the motion segment unit beyond normal constraints. Excess motion can be abnormal in quality (i.e. abnormal coupling patterns) or in quantity (abnormal increased motion) or both. Excess motion results in damage to the nerve roots, the spinal cord, and other spinal structures.
The underlying causes of the structural changes in the motion segment unit leading to instability are trauma, degeneration, aging, disease (tumor, infection, etc.), surgery, or a combination thereof. It is known that a mechanically unstable motion segment unit can originate due to degeneration of the nucleus pulposus. A degenerate nucleus leads to disc space narrowing, loss of viscoelastic properties and the subsequent transfer of compressive loads to the annulus fibrosus. The altered anatomic dimensions and subsequent abnormal response to loading can cause loss of pre-tension in the ligamentum flavum, and longitudinal ligaments, degeneration of the facet capsules (and possible subluxation) with a consequence of secondary degenerative osteoarthritis of the joints.
Current surgical techniques employed in spine surgery require the removal of ligaments and bone, in addition to sections of the intervertebral disc. The result of such procedures diminish the structural integrity of the spine joint. As can be seen by the work of D. A. Nagel et al., "Stability of the Upper Lumbar Spine Following Progressive Disruptions and the Application of Individual Internal and External Fixation Devices", JBJS Vol. 63-A, No 1, January, 1981, pp. 62-70, disruption of the supraspinous and interspinous ligaments, ligamentum flavum, and the facets at L1-2 increases the range of motion in Flexion-Extension, Lateral Bending, and Axial Rotation by 4.1 degrees (48.8% increase), 0.8 degrees (16.32% increase), and 1.4 degrees (63.63% increase) respectively. Disruption of the posterior longitudinal ligament, posterior annulus, and lateral annulus in addition to the above, increase the average range of motion in Flexion-Extension, Lateral Bending, and Rotation by 8.9 degrees (105.95% increase), 3.4 degrees (69.39% increase) and 12.4 degrees (563% increase) respectively over the intact case alone.
An unstable motion segment unit may be fused to form a permanent or rigid internal fixation of all or part of the intervertebral joints using such materials as rods, hooks, metallic mesh, plates, bolts, screws and/or cement. However, permanent spinal fixation is a difficult surgical technique due to the irregular shape of the bones, the relative weakness of most of the bones of the vertebrae and the complexity and strength of the deforming muscular forces acting on the trunk.
The need for a compliant or flexible spinal implant is evidenced in three prominent factors: (i) reoperations required for patients having undergone spinal decompressive surgery (including discectomies), (ii) further MSU degeneration in patients with fusions and internal fixation, and (iii) MSU instability resulting from the surgical procedure, in patients not normally indicated for fusion.
The re-operation of patients having undergone spinal decompressive surgery is of concern as a significant number of long-term failures may be evidenced. For example, J. W. Frymoyer et al., "Segmental Instability: Rational for Treatment", Spine, 10:280-287, 1985, undertook a study to compare the long-term effects of surgery on lumbar disc disease when treated by disc excision alone or by disc excision combined with primary posterior midline fusion. The study demonstrated a high percentage of unsatisfactory results in patients who had either simple disc excision or disc excision combined with spinal fusion. Thirty percent (30%) of patients whose spines were fused and 38% of those patients whose spines were not fused were considered long-term failures because of persistent symptoms or the need for reoperation.
In addition to the long-term failures of disc excision with or without fusion, the use of internal fixation with fusion following disc excision has shown no increase in success rate. A. A. White et al., Clinical Biomechanics of the Spines, Philadelphia, J. P. Lipponcot, Co., 1978, reported on the first ten years of a prospective study on herniated lumbar disc patients who underwent surgery. The results indicated that the addition of a fusion with internal fixation after a bilateral laminectomy and disc excision did not increase the subjective or objective success rate. Fair to poor results were seen in 22% of the patients with no fusion and 42% of the patients with fusion.
In lieu of the success/failure rates of these procedures, other underlying mechanical phenomena may contribute to the further degeneration of the spine's motion segment units. Radiographic findings suggest that spinal fusion imposes new stresses on the vertebral motion segment above the fusion. R. Quinnell et al., "Pressure Standardized Lumbar Discography", British Journal of Radiology, 53:1031-1036, 1980, and C. K. Lee et al., "Lumbosacral Spine Fusion--A Biomechanical Study", Spine Vol. 9, No. 6, 1984, pp. 574-581, found alterations of the mechanics at adjacent levels when experimental floating lumbar fusions were performed on cadaver spines. The concern about increased mechanical stress at adjacent motion segments has been reinforced by clinical reports of lumbar spinal stenosis at the motion segment immediately above lower lumbar spine fusions and acquired spondylolysis at the cephalad vertebra in the fusion mass. Therefore, fusion generates a conflict between immediate benefit and late consequences.
In the course of surgical management of a herniated disc, the surgeon must dissect the supraspinous and interspinous ligaments as well as other soft tissue to expose and remove the herniated mass. While no studies have been organized to determine the contribution of surgically induced MSU instability, data concerning the incidence of re-operation at the same level in those patients not fused at the time of surgery indicates there is a relationship. J. Dvorak et al., "The Outcome of Surgery for Lumbar Disc Herniation--I. A 4-17 Years' Follow-up with Emphasis on Somatic Aspects", Spine Vol. 13, No. 12, pp. 1418-1422, 1988, found that in 362 patients follow-up 4-17 years after surgery indicated almost 50% of the patients considered their long-term results unsatisfactory. Moreover, A. A. White, "Overview of and Clinical Perspective on Low-Back Pain Syndrome", Spine Update 1984, edited by HK Genant, San Francisco, Radiology Research and Education Foundation, 1984, pp. 127-130, estimates the international average for recurrence at the same lumbar level at 15%.
The current management of spinal fusion may include the use of rigid metallic rods and plates. These systems have been used regularly since the early 1960's, first for the management of scoliosis, and then for the management of low-back disease. Current systems are attached either by means of a hook or by pedicle screws. In all cases, the device is intended to rigidly immobilize the motion segment unit to promote fusion. Due to its inherent rigid design (as compared to the surrounding bone), these devices have often caused localized osteoporosis at the attachment sites P. C. McCaffee, "Device Related Osteoporosis With Spinal Instrumentation", Spine, 14(9), pp. 919-926, (stress shielding due to the rigidness of the implant), and have directly and indirectly contributed to the degeneration of the joints above and below the fusion site as well as at the fusion site itself (see R. Quinell et al. and C. K. Lee et al.). Due to their material composition, these stainless steel devices have frequently been rejected by patients in response to the release of metal ions. Furthermore, the ferromagnetic properties of the implant material itself has prevented the use of post-operative MRI or CT scan imaging due to scatter of the image.
In the design of such implants, several criteria must be taken into consideration. These criteria include the modulus of the implant material, the geometry and dimensions of the device as well as the biocompatibility or inertness of the implant material. The modulus of elasticity of the material may be expressed as the ratio of material stress (force per unit area) to strain (% elongation). Materials with a higher modulus will exhibit less elongation when exposed to the same stress than those materials of lower modulus and as such will appear stiffer.
The geometry and dimensions of the device will dictate implant performance in accordance with accepted mathematical concepts. Implant geometry will be used in describing the mathematical model of the implant. Implant dimensions will be used to help quantitate the model. For example, implant dimensions will help quantitate the cross-sectional moment of inertia which can be described as .pi.r.sup.4 /4 for a rod-like element.
The biocompatibility of the implant material is essential in implant design as to prevent the "poisoning" of the patient as well as to prevent a biological response which may corrode the implant material. Not only must the material be biocompatible, but it must be suited for load bearing applications in the body.
Examples of proposed spinal implants include Burton, U.S. Pat. No. 4,743,262, which discloses a stabilization system for a vertebral column in which the posterior vertebral elements are removed to enable attachment of bar-like elements to adjacent vertebra. The supporting bars can be fabricated from a carbon reinforced plastic.
While providing some flexibility and support, the device disclosed in Burton is disadvantageous because it removes the posterior elements (facet capsules) which provide about 20% of the support inherent of the spine as well as torsional stability for the joint. Removal of the posterior elements reduces the amount of support available for the affected motion segment unit. In addition, linear bar-like elements cannot provide support and movement which closely approximates the function of the motion segment unit.
Other examples include Brantigan, U.S. Pat. Nos. 4,834,757 and 4,878,915, where systems are disclosed for the support of the vertebral column through the use of plugs to be placed in the disc space. The first device disclosed by Brantigan is a biocompatible composite cage whose intended use is to contain either autologous or allograft bone and promote fusion of the vertebral bodies.
While the device disclosed by the Brantigan '757 patent will provide compressive support to the spinal column, it is a rigid support and as such does not allow for normal joint motion. Moreover, the role of the device diminishes as the surrounding vertebral bone integrates into the bone contained within the cage.
The Brantigan '915 patent discloses a solid device having barbs for biting into the bone as well as spaces between the barbs intended to be sites of bone ingrowth. Again, as in the '757 patent, the aforementioned device is intended to promote fusion of vertebral bodies thereby eliminating any motion within that spine joint.
Therefore, based on clinical evidence suggesting the incidence of long-term failures and incidence of further degeneration at the levels adjacent to the fusion site, and the limitations posed by using rigid metallic systems, there is a need for an implantable prosthetic device which can restore normal biomechanical function to an injured or diseased motion segment unit by reducing the load on the existing vertebral disc and facet joints or capsules. Such a device would be compliant or flexible and allow for joint motion in six-degrees of freedom, yet, would limit motion beyond that which has been determined to be unphysiologic.